High surface area materials have attracted much attention for their potential uses in electrocatalysis, batteries, fuel cells, and sensors. (See Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W., Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nature Materials 2005, 4, (5), 366-377; Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A., Carbon Nanotubes—The Route Toward Applications. Science 2002, 297, (5582), 787-792; Hutchings, G. J.; Haruta, M., A Golden Age Of Catalysis: A Perspective. Applied Catalysis A-General 2005, 291, (1-2), 2-5; Jurczakowski, R.; Hitz, C.; Lasia, A., Impedance of Porous Au Based Electrodes. Journal Of Electroanalytical Chemistry 2004, 572, (2), 355-366; Wang, J., Carbon-Nanotube Based Electrochemical Biosensors: A Review. Electroanalysis 2005, 17, (1), 7-14.) For materials to be useful in these applications they must have large, electrochemically active surface areas. To maintain current trends in minimizations, however, these large surface areas need to be contained in a small volume. Thus, high density is needed in addition to high surface area. Creating high density, high surface area materials that are fully electrochemically active can be a difficult task. As a result, much attention has been given to nanoscale carbon-based materials due to their high surface areas, ability to create high density arrangements and unique chemical and physical properties. (See Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A., Carbon Nanotubes—The Route Toward Applications. Science 2002, 297, (5582), 787-792; Dai, H. J., Carbon Nanotubes: Opportunities and Challenges. Surface Science 2002, 500, (1-3), 218-241; Harris, P. J. F., Carbon Nanotube Composites. International Materials Reviews 2004, 49, (1), 31-43.) Nanoscale carbon materials are also attractive because they can be combined with metal nanoparticles, or conducting polymers, to form composites with tailored electrical properties.
A particularly interesting class of carbon nanotubes are the vertically aligned carbon nanofibers (VACNFs). Vertically aligned carbon nanofibers are multi-walled carbon nanotubes that are typically grown in a DC plasma, yielding nanofibers that are aligned vertically from the surface. (See Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Provencio, P. N. Science 1998, 282, 1105.) The resulting nanofiber “forests” have interesting properties because in addition to providing edge planes along the nanofiber walls, the interstices between the fibers are straight and relatively large, providing a high degree of accessibility to analytes. The presence of well-defined interstices is important because previous studies of many nanostructured, high surface-area carbon materials have found that very small pores cannot support electrical double-layers and diffusion limitations can reduce the effective surface area. (See Frackowiak, E.; Beguin, F. Carbon 2001, 39, 937; de Levie, R. Electrochimica Acta 1964, 9, 1231.)
Vertically aligned carbon nanofibers are a promising high surface area, nanoscale carbon material. Vertically aligned carbon nanofibers have similar electrochemical and mechanical properties as other nanoscale carbon materials. The advantage of VACNFs is the ability to control their physical dimensions allowing for large, accessible surface areas. Thus, VACNFs are an ideal platform for modifications leading to increased surface area, such as covalent functionalization with molecular layers and decoration with metal coatings.
Carbon-based nanostructures have been decorated with metal coatings, but the methods employed, the underlying carbon platforms used, and the resulting coated materials suffer from a number of significant drawbacks. Vapor deposition and electrochemical methods have been used to coat nanoporous materials with metals, but these processes clog pores, limiting the surface area of the coated substrates. In contrast, electroless deposition is a more favorable coating technique. In previous studies, however, electroless deposition of metals has involved oxidation of the underlying carbon nanostructures. The relatively harsh oxidation conditions used are both detrimental to the nanostructures and difficult to integrate with the underlying metal electrodes. Furthermore, metal coatings derived from these methods are not uniform and continuous, but consist of discrete metal nanoparticles attached to the surface of the carbon substrates. Finally, the carbon platforms used in these studies have been free-standing, unattached single-walled or multi-walled carbon nanotubes arranged in a spaghetti-like mat. Unlike VACNFs, such structures are not ideal platforms for high surface area electrodes. Therefore, a need exists for a method of decorating carbon nanostructures with uniform metal coatings to provide electrodes with high structural stability and very high surface areas.